1. Technical Field
The present invention relates to semiconductor processing techniques, and more particularly, techniques for fabricating semiconductors suitable for use at thin-film transistor (“TFT”) devices.
2. Background Art
Semiconductor films, such as silicon films, are known to be used for providing pixels for liquid crystal display devices and organic light emitting diode display devices. Such films are commonly processed via excimer laser annealing (“ELA”) methods, where an amorphous silicon film is irradiated by an excimer laser to be crystallized.
Significant effort has gone into the refinement of “conventional” ELA (also known as line-beam ELA) processes in the attempt to improve the performance of the TFT devices placed on the processed semiconductor thin films. For example, U.S. Pat. No. 5,766,989 issued to Maegawa et al., the entire disclosure of which is incorporated herein in its entirety by reference, describes the ELA methods for forming polycrystalline thin film and a method for fabricating a TFT. The '989 patent attempts to address the problem of non-uniformity of characteristics across the substrate, and provide certain options for apparently suppressing such non-uniformities.
However, the details of the beam-shaping approach used in conventional ELA methods make it extremely difficult to reduce the non-uniformities in the semiconductor films and to improve the performance characteristics of such films. For example, in a low-temperature polycrystalline silicon (“LTPS”) process, when the size of the grains becomes comparable to the dimensions of the channel region of the TFT, large device-to-device non-uniformity results. This is caused by the randomness of the microstructure, i.e., the random location of the grains and thus the grain boundaries. Such non-uniformity, especially when perpendicular to the current flow, can act as a current barrier. Further, when the transistor is in its off-state, carriers are generated at the grain boundary, which contribute to the off-current. This is especially the case when the grain boundary is in or close to the drain-channel junction.
Therefore, it has been realized that control over the microstructure is needed in order to ensure a uniform TFT process, both with respect to periodicity and location. Regarding the former, the film should be uniform, exhibiting periodicity in the location of the grains and thus the grain boundaries. Regarding the latter, the location of the grains and thus the grain boundaries should be controlled so that their contribution to the electrical characteristics is the same for every single device.
In an pulsed-laser, e.g., an excimer laser, irradiaton process to obtain LTPS films, control over the TFT microstructure may be obtained through the use lithography to induce such periodicity. The use of lithography also accounts for location control, since the accurate alignment procedure of the lithographic process is used. Unfortunately, the use of lithography requires at lease one extra processing step, which in turn increases complexity and thus costs.
Alternatively, control over the TFT microstructure may be obtained through the use of sequential lateral solidification (“SLS”) techniques. For example, in U.S. Pat. No. 6,322,625 issued to Im and U.S. patent application Ser. No. 09/390,537 (the “'537 application”), which is assigned to the common assignee of the present application, the entire disclosures of which are incorporated herein by reference, particularly advantageous apparatus and methods for growing large grained polycrystalline or single crystal silicon structures using energy-controllable laser pulses and small-scale translation of a silicon sample to implement sequential lateral solidification have been described. As described in these patent documents, at least portions of the semiconductor film on a substrate are irradiated with a suitable radiation pulse to completely melt such portions of the film throughout their thickness. In this manner, when the molten semiconductor material solidifies, a crystalline structure grows into the solidifying portions from selected areas of the semiconductor film which did not undergo a complete melting. Thereafter, the beam pulses irradiate slightly offset from the crystallized areas so that the grain structure extends into the molten areas from the crystallized areas.
Using the system shown in FIG. 1, an amorphous silicon thin film sample is processed into a single or polycrystalline silicon thin film by generating a plurality of excimer laser pulses of a predetermined fluence, controllably modulating the fluence of the excimer laser pulses, homogenizing the modulated laser pulses in a predetermined plane, masking portions of the homogenized modulated laser pulses into patterned beamlets, irradiating an amorphous silicon thin film sample with the patterned beamlets to effect melting of portions thereof corresponding to the beamlets, and controllably translating the sample with respect to the patterned beamlets and with respect to the controlled modulation to thereby process the amorphous silicon thin film sample into a single or polycrystalline silicon thin film by sequential translation of the sample relative to the patterned beamlets and irradiation of the sample by patterned beamlets of varying fluence at corresponding sequential locations thereon.
While the system of FIG. 1 is highly advantageous in generating uniform, high quality polycrystalline silicon and single crystal silicon which exhibit periodicity and thereby solves a problem inherent with conventional ELC techniques, the technique does adequately not account for control over grain boundaries. For example, in the simplest form, SLS requires two pulses to crystallize the amorphous precursor into an LTPS film with partial periodicity, e.g., the 2-shot material shown schematically in FIG. 2a. The periodicity is only in one direction, shown by long grain boundaries 210, 220, 230, 240, 250 that are parallel to each other and which also have a protrusion to them. However, the position of the short grain boundaries is not at all controlled. The spacing between the parallel grain boundaries can be increased, and this material is in general called n-shot material. Likewise, FIG. 2b shows a so-called 4-shot material in which the grain boundaries are periodic in both directions. Again, the spacing between the grain boundaries can be increased, and is generally referred to as 2n-shot material.
While SLS techniques offer periodicity, such techniques do not offer accurate control of the location of grain boundaries. Referring to FIGS. 2c–d, the LTPS film produced includes a varying number of long grain boundaries perpendicular to the current flow, and the possibility of having a perpendicular grain boundary in or out of a TFT drain region. Both problems become more severe when grain size is increasing and/or when channel dimensions are decreasing, i.e., when the size of the grains becomes comparable to the dimensions of the channel region. While there has been a suggestion in U.S. Pat. No. 6,177,301 to Jung to misalign TFT channel regions with respect to the grain growth direction, that suggestion is made without taking into account the underlying need to maintain uniformity in TFT microstructure. Accordingly, there exists a need for a TFT manufacturing technique that provides for control over both the periodicity of grain boundaries and the location of TFTs in order to provide for uniformity in TFT microstructure.